The present invention relates on one hand to a method as defined in the preamble of claim 1, and on the other hand to a system as defined in claim 8.
Such method is adapted to control the cooling of small milk quantities (milk spots) entering consecutively and with time intervals a cooling tank in which there are arranged agitator means and at least one milk contacting cooling surface being part of a cooling circuit.
A system suitable for carrying out such method includes a cooling tank containing an agitator means and a milk cooling surface being part of an evaporator in a cooling circuit.
An automatic milking system (AMS) allows dairy cows to be milked voluntarily or on demand with little or no human interaction. In a milking machine used in an AMS the milking is carried out per individual quarter of the milked cow""s udder. To minimize the bacterial growth and contents of free fatty acids (FFA) in milk extracted from a cow being milked by a quarter milking arrangement it is highly desirable to cool down the milk as soon as possible before or at the latest in the refrigerated bulk tank. In an AMS relatively small quantities of milk occasionally will have to be conveyed to the bulk tank. This is the case particularly during the night when there might be periods of considerable duration with only a low flow of milk because of a low activity of the cows for milking. Long delivery lines for the milk from the teat cups to the bulk tank will of course further emphasize the necessity of effectively cooling the milk as soon as it enters the bulk tank, or the first refrigerated tank in the system.
In some cases it may be appropriate to use a collecting pre-cooling auxiliary tank upstream of the refrigerated bulk tank.
However, in view of the occasionally small milk quantities (milk spots) entering such the cooling tank (which may be the usual refrigerated bulk tank or some other cooling tank) there is an obvious risk that the milk spots entering such nearly empty cooling tank (direct expansion) will be freezing on the cooling surfaces thereof. Such freezing of the milk will have a detrimental effect on the milk quality.
A primary object of the invention is to maintain a high milk quality by avoiding the risk of milk spot freezing on cooling surfaces in an empty or nearly empty cooling tank (either a refrigerated bulk tank or some other type of tank), especially when the milk is entering such tank in small quantities (milk spots) fed into the tank with considerable time intervals between said milk quantities (spots).
Thus, in other words the problem is to be able to cool down a small milk quantity, e.g. a thin milk layer, on the bottom of a milk cooling tank without any risk of ice formation in the milk, since such ice formation will deteriorate the milk quality. This problem is further accentuated in case an effective stirring of the milk is not possible, e.g. because of a faulty stirrer (agitator means) or since the milk quantity is too small (e.g. too shallow milk layer) for the stirrer to be able to work satisfactory.
A secondary object of the invention is to be able to use a smaller and cheaper cooling apparatus having a smaller cooling capacity than present cooling tanks, but nevertheless, when necessary, a quite sufficient capacity to be able to cool the flow of milk from a number of high-producing cows, using the milking system in rapid succession.
The objects of the invention are achieved by utilizing the cooling method defined in claim 1, and by means of the system defined in claim 8. Steps and features adapted to elaborate the invention are provided in claims 2-7 and 9-10, respectively.
The method according to the invention is characterized by the following main steps:
a) providing a means for measuring the milk quantity in the tank, and a temperature transducer for monitoring the milk temperature in the tank,
b) providing, in series in the cooling circuit, an evaporator connected to the bottom wall portion of the tank, a compressor, and a condenser,
c) controlling the temperature of the refrigerant in the evaporator by regulating the vaporising pressure, so that the temperature of the milk contacting cooling surface is always at least slightly higher than 0xc2x0 C., whereas the refrigerant temperature in the evaporator is below 0xc2x0 C. when the compressor is running and,
d) monitoring, by said measuring means, the milk quantity in the tank, and, when said milk quantity turns out to be sufficient/insufficient for the agitator means to work properly, starting/stopping the operation of the agitator means.
The temperature transducer (which is adapted to monitor the milk temperature in the tank) may be arranged within the tank or on the outside of the tank. It is not the location of the temperature transducer per se that matters, but its ability to provide a representative value of the milk temperature inside the tank.
If the temperature transducer is to be arranged on the outside of the tank, a suitable position would be on the outer surface of the tank wall, preferably on the outside of the tank bottom wall at a distance from the evaporator.
The temperature measuring transducer may be any suitable type of electric/electronic temperature measuring device.
The evaporator, which is connected to the bottom wall portion of the tank, may comprise refrigerant conveying channels between the outside of the bottom wall portion of the tank and the inside of a sheet casing welded to said wall portion on the outside thereof. The refrigerant is vaporized (at a low pressure) in these channels by means of heat supplied by the milk inside the tank. A refrigerant temperature below 0xc2x0 C. in the evaporator will actually be obtained only when the compressor has been running for a while.
The gist of the invention is to prevent ice formation in the milk at the bottom of the tank by monitoring and controlling the pressure of the refrigerant in the evaporator.
From a general point of view the milk cooling process mainly depends on: the milk quantity in the tank; the size of the milk cooling surface; the temperature difference between the temperature of the milk to be cooled and the vaporising temperature of the refrigerant in the evaporator; and the flow of the milk over the cooling surface.
Various steps for obtaining the desired control characteristics are defined in the dependent claims 2-7.
In view of the fact that the milk supplied to the cooling tank enters same in the form of small quantities or spots with time intervals (i.e. spread over the 24 hours of a day and night) the cooling circuit apparatus (compressor, condenser and evaporator) can be a small and cheap unit having a rather limited cooling capacity.
Since the evaporation temperature (i.e. vaporising temperature) depends on the pressure in the evaporator, it is desirable to be able to control that pressure.
Furthermore, according to the invention a system (i.e. arrangement) for controlled cooling of milk spots is defined by the features of claim 8.
In such a system, the cooling circuit comprises in series therein an evaporator, a motor-driven compressor being connected to a refrigerant outlet from the evaporator and, through an interconnected condenser and an expansion valve downstream thereof, to a refrigerant inlet to the evaporator. A milk quantity measuring means and a temperature transducer (for monitoring the milk temperature in the tank) are provided in the system. The temperature transducer may be positioned within the tank or on the outside thereof, and the milk quantity measuring means may be placed in a part of the milking equipment upstream of the tank, or in a wall section of the tank, or even within the tank.
Preferably, a control equipment is connected to the temperature transducer (for receiving a temperature input) and also to the motor driving the compressor for regulating same. A temperature sensor, which is connected between the evaporator outlet and the compressor inlet and senses the temperature of the vapour flow there between, serves to control the expansion valve.
Furthermore, a pressure sensor, which is connected between the temperature sensor and compressor inlet and serves to sense the pressure of the vapour, submits an output signal to the control equipment.